1. Field of the Invention
The present invention relates to a FBAR (Film Bulk Acoustic Resonator), and more particularly to a FBAR device comprising a lower electrode film with crystalline characteristics required to form an excellent piezoelectric film and improved electrode film characteristics, and a method for producing the same.
2. Description of the Related Art
In order to meet the rapid development of communication techniques, there has been required the development of signal processing and RF (Radio Frequency) component manufacturing techniques. Particularly, the RF component manufacturing techniques have been developed such that filter components are replaced with FBAR filters so as to meet miniaturization trends of mobile communication units and radio sets.
A FBAR device has a fundamental structure in which an electrode is formed on upper and lower surfaces of a piezoelectric layer on an air gap. When voltage is applied to the upper and lower electrodes, a part of electric energy is converted to mechanical energy such as an acoustic wave by resonance characteristics of the piezoelectric layer. Thereby, the FBAR device is operated as a filter.
Generally, a FBAR device is formed on a substrate. The substrate includes various isolation structures for protecting the substrate from acoustic waves generated from a piezoelectric layer of the FBAR device. For example, the substrate of the FBAR device may have a certain space formed at an area corresponding to a resonance generating position shown in FIG. 1, i.e., an air gap, or have a reflective layer using Bragg reflection.
FIG. 1 is a cross-sectional view of a conventional FBAR (Film Bulk Acoustic Resonator) device using an air gap.
As shown in FIG. 1, the FBAR device comprises a substrate structure 10, and an acoustic resonant portion 20 including a lower electrode film 22, a piezoelectric layer 24 and an upper electrode film 26 formed on the substrate structure 10 in sequence. As shown in FIG. 1, the substrate structure 10 includes a silicon substrate 11 and an air gap 15 formed on the upper surface of the silicon substrate 11. The air gap 15 of the substrate structure 100 is obtained by forming a cavity in the upper surface of the substrate 11, filling the cavity with a sacrificial layer, filling the acoustic resonant portion 20 with the sacrificial layer, and removing the sacrificial layer through via holes.
Generally, the piezoelectric layer 24 is made of aluminum nitride, i.e., AlN, and the lower and upper electrode films 22 and 26 are made of molybdenum (Mo). Characteristics of the FBAR device are determined by the piezoelectric layer 24 and the lower and upper electrode films 22 and 26, and particularly resonance characteristics of the piezoelectric layer 24 is a leading factor for determining a Q value of the FBAR device.
In order to obtain excellent resonance characteristics of the FBAR device, the AlN layer of the piezoelectric layer 24 is grown so that it has a preference for (002) orientation. The crystalline characteristics of the piezoelectric layer 24 are much dependent on the crystalline characteristics of a lower electrode. That is, the resonance characteristics of the FBAR device depend on the crystalline structure of the Mo lower electrode film 26. Accordingly, in order to obtain a preference for (002) orientation of the AlN layer, the lower electrode film 22 requires a preference for (110) orientation during its growth.
Conventionally, in order to obtain the above crystalline characteristics of the Mo lower electrode film, a method for improving the conditions in a depositing step of the lower electrode film has been used. For example, the crystalline characteristics of the Mo lower electrode film are changed by raising the sputtering power or the temperature of the substrate in the depositing step or lowering the partial pressure of argon (Ar) in a Mo sputtering step for forming the lower electrode film. However, it is difficult to obtain sufficient crystalline characteristics of the Mo lower electrode film only by changing the conditions of the sputtering step, and the control of the change of the conditions complicates the process.
Further, the change of the conditions undesirably influences the Mo electric film, in the process for improving the crystalline characteristics of the Mo electrode film. For example, the raising of the sputtering power improves the crystalline characteristics of the Mo electrode film, but causes exfoliation of the Mo electrode film from the substrate due to stress generated in the sputtering step.
Moreover, the lower electrode film grown on the substrate in the conventional FBAR device cannot have excellent electrode characteristics.
FIGS. 2a and 2b are photographs, each illustrating the surface state and the sectional structure of the Mo lower electrode film of the conventional FBAR device, taken by means of a SEM (Scanning Electron Microscope).
With reference to FIGS. 2a and 2b, the surface state and the sectional structure of the Mo lower electrode film formed on a silicon nitride layer of the substrate structure are shown. The surface of the Mo lower electrode film is somewhat rough (Ra>10Å), and the sectional structure of the Mo lower electrode film is not so dense. This structure of the Mo lower electrode film depreciates the resonance characteristics of the FBAR device, and causes problems such as comparatively large resistivity and poor intensity. Since these problems vary sensitively according to the depositing conditions of the Mo lower electrode film, particularly according to the variation of the partial pressure of argon gas, it is difficult to control process conditions.
Accordingly, there has been required a technique for forming a Mo lower electrode with excellent crystalline characteristics so that the resonance characteristics of the piezoelectric layer and the electrode film characteristics of the Mo lower electrode film are improved regardless of the Mo depositing conditions by sputtering.